Apparatus and method for fatigue testing

Abstract

A method of simulating life of a rotor blade mounting feature mounted in a complementary rotor disc mounting feature comprises the steps of providing two specimen mounting features (37) each having portions defining pressure faces matching in profile respective faces of the rotor blade mounting feature; providing a fixture (38) having two opposed mounting features (39) each having portions defining pressure faces (78) matching in profile respective faces of the rotor disc mounting feature; mounting the specimen mounting features (37) in the fixture mounting features (39); applying a first load to the specimen mounting features (37); measuring a strain in a region (86) of the fixture (38) between the fixture mounting features (39); applying a second load to the fixture (38), the second load being substantially perpendicular to the first load; and controlling the second load in response to the measured strain. A high cycle load may simultaneously be applied to the specimen mounting features (37). Claims are also included for an apparatus (10) for carrying out the method.

Description

FIELD OF THE INVENTION[0001]This invention relates to fatigue testing of materials, and more particularly relates to an apparatus and method for simulating fatigue of rotor blade roots in complementary rotor blade slots.BACKGROUND OF THE INVENTION[0002]Gas turbine engine fan blades, compressor blades and turbine blades are subjected to a combination of low cycle fatigue (LCF) and high cycle fatigue (HCF) stresses during the engine's operation. These LCF and HCF stresses have a detrimental effect on the integrity of the blades. The LCF stresses result from the centripetal force experienced by the blades as they rotate about the engine axis. The HCF stresses result from aerodynamic and other vibration excitation of the blades.[0003]In order to design fan blades, compressor blades and turbine blades that are resistant to fatigue, a good understanding is required of the combination of the steady and alternating stresses to which a blade may be subjected in operation.[0004]The fatigue te...

AI Technical Summary

Benefits of technology

[0011]Accordingly the present invention seeks to provide a novel apparatus and method for the fatigue testing of materials which reduces, and preferably overcomes, the problems set out above.

Problems solved by technology

These LCF and HCF stresses have a detrimental effect on the integrity of the blades.

The fatigue testing of materials under conditions representative of gas turbine operating conditions is difficult to achieve for blade aerofoil shapes and blade root shapes.

It is known to perform tests on specimens representative of blade shapes, but the behaviour of these specimens does not accurately reflect the behaviour of real blades.

During service operation, the rotor blade assembly is subject to a complex loading system, comprising centripetal load, gas load and vibration.

Rotation of the fan assembly results in a large load on the dovetail root, due to centripetal acceleration, as the blade tries to pull out of the retention slot.

In addition, as the fan blade compresses the incoming air, the pressure differential across the blade causes the aerofoil to bend, imparting an additional bending load into the dovetail root.

Under the action of these loads, sliding occurs between the blade and disc dovetail, which in combination with the high contact pressures between the two components, can lead to rapid loss of traditional low-friction dry film lubricants.

Further wear of the coating leads to metal-to-metal contact between the blade and disc, resulting in heavy frettage and wear of the underlying material.

In some cases, interfacial cold welding and plucking can occur, leading to heavy galling and the generation of additional stress concentration features.

These processes, in combination with the applied loads, can lead to the initiation of fatigue cracking at the dovetail edge-of-contact.

The underlying component stress field due to centripetal load, gas bending and vibration may then propagate these cracks, resulting in blade or disc failure.

In the worst case, where vibration stress levels are high enough to cause crack propagation, failure may occur in a very small number of flights.

Specifically, the friction level on the dovetail contact flank influences fatigue life.

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